Coiled steel tubing finds a number of uses in oil well operations. For example, it is used in running wireline cable down hole with well tools, such as logging tools and perforating tools. Such tubing is also used in the workover of wells, to deliver various chemicals downhole and perform other functions.
Steel coiled tubing is capable of being spooled because the steel used in the product exhibits high ductility (i.e. the ability to plastically deform without failure). The spooling operation is commonly conducted while the tube is under high internal pressure which introduces combined load effects. Unfortunately, repeated spooling and use causes fatigue damage and the steel coiled tubing can suddenly fracture and fail. The hazards of the operation and the risk to personal and the high economic cost of failure in down time to conduct fishing operations forces the product to be retired before any expected failure after a relatively few number of trips into a well. The cross section of steel tubing expands during repeated use resulting in reduced wall thickness and higher bending strains with associated reduction in the pressure carrying capability. Steel coiled tubing is limited as to internal pressures up to about 5000 psi. Higher pressures significantly reduce the integrity of the tubing so that it will not sustain continuous flexing and thus severely limit its life, even to a single field application.
It is therefore desirable to provide a non-steel coil tubing which is capable of being deployed and spooled under borehole conditions, which does not suffer from the limitations of steel tubing and is highly resistant to chemicals.
Additionally, present steel coiled tubing logging operations sometimes utilize a wireline cable inserted within the bore of the coiled tubing to transmit data, or when real time data is not required, a logging tool capable of collecting and storing data downhole. When real time data is required, a dedicated reel of coiled tubing is used with the wireline permanently installed in the tubing. This takes up substantial cross-sectional space within the tubing and thus renders the coiled tubing unsuitable for other operations requiring a flow path or open bore through the tubing. Fluids are sometimes transported from the surface to a downhole location through the tubing to provide means for treating formations or for operating a mud motor to drill through the formations. In addition, it may be desirable to pump devices through the coiled tubing bore to a downhole location for various operations. Therefore, an open bore within the coiled tubing is essential for many operations and for this reason it is desirable not to have electrical conductors or the like positioned within the open bore portion of the tubing.
External pressures on the coiled tubing are also a major load condition and can be in excess of 2500 psi.